1. Field of the Invention
The present invention relates to a thin film magnetic head. Specifically, it relates to a method for manufacturing a thin film magnetic head provided with a pair of magnetization free layers whose magnetization directions vary according to an external magnetic field.
2. Description of the Related Art
Conventionally, a spin valve head is known as a head having high power and high sensitivity and used for a hard disk drive (HDD). In order to fix a magnetization direction of one ferromagnetic layer of a pair of ferromagnetic layers that are disposed on both sides of a non-magnetic intermediate layer, an antiferromagnetic layer made of a material such as IrMn is used for the spin valve head. Since the antiferromagnetic layer has a relatively thicker film thickness, and may restrict further high recording density (narrowing of a read gap), a new concept attempts to narrow a read gap. In this specification, the read gap refers to a gap between upper and lower shield layers.
A thin film magnetic head is disclosed in the specification of U.S. Patent Application Publication No. 2009/0034132. The thin film magnetic head has two magnetization free layers (upper and lower magnetization free layers) whose magnetization directions vary according to an external magnetic field, and a non-magnetic intermediate layer that is sandwiched by the two magnetization free layers. In this specification, a magneto-resistance (MR) element having the above-described configuration may be indicated as having dual free layers (DFL). The two magnetization free layers are exchange-coupled based on RKKY (Rudermann, Kittel, Kasuya, and Yoshida) interaction through the non-magnetic intermediate layer. The two magnetization free layers are magnetized in antiparallel directions to each other under the state that a magnetic field is not applied at all. In this specification, “antiparallel” means that magnetization directions are parallel but opposed to each other. A bias magnetic layer is disposed on rear sides of the two magnetization free layers, seen from an air bearing surface (ABS), and a bias magnetic field is applied in an orthogonal direction to the ABS. Magnetization directions of the two magnetization free layers form a certain relative angle by the magnetic field generated from the bias magnetic layer. Under this state, when an external magnetic field, which is in an orthogonal direction to the ABS, is applied from a recording medium, the magnetization directions of the two magnetization free layers vary, the relative angle between the magnetization directions of the two magnetization free layers varies, and an electrical resistance of a sense current varies. Therefore, the external magnetic field may be detected by using the above-described property.
Since the film configuration configured with the two magnetization free layers needs no antiferromagnetic layer, the film configuration is simplified. As a result, it becomes easier to narrow the read gap. Specifically, the following configuration has an advantage: exchange coupling layers (upper and lower exchange coupling layers) formed of, for example, ruthenium are disposed in an outside of the two magnetization free layers, and magnetization control layers (upper and lower magnetization control layers) whose magnetization directions are fixed are disposed outside the exchange coupling layers as parts of upper and lower shield layers. In this configuration, exchange coupling through the exchange coupling layers magnetizes the two magnetization free layers in antiparallel directions to each other under a state where the magnetic field is not applied at all. Since the magnetization control layers are disposed as a part of the shield layer, it does not configure the read gap, and it is favorable to narrow the read gap.
In order to manufacture the thin film magnetic head having the above described configuration, a lower shield layer is formed, several layers including the two magnetization free layers and the two exchange coupling layers are sequentially formed above the lower shield layer, and these layers are formed in a pillar shape. These layers are formed in the pillar shape by disposing a resist on the formed layer, and applying a known method such as ion beam etching. In this case, a non-magnetic layer (a cap layer) made of Ta, Ru, or the like is formed on an upper exchange coupling layer due to photoprocess. However, when a pillar forming process is finished, the non-magnetic layer needs to be completely removed. Because, if the non-magnetic layer remained, not enough exchange coupling strength would be generated between the upper magnetization control layer and the upper magnetization free layer.
When the non-magnetic layer is completely removed, the upper exchange coupling layer positioned directly thereunder receives significant damage. Therefore, it is desirable that a passivation layer for the upper exchange coupling layer is disposed between the non-magnetic layer and the upper exchange coupling layer. The passivation layer is preferably formed with a NiFe layer. The passivation layer not only protects the upper exchange coupling layer, but also magnetically integrates the upper magnetization control layer laminated thereon. The passivation layer functions as a part of the upper magnetization control layer during operation of the head. Hereafter, the passivation layer is designated as an auxiliary magnetization control layer. In such a configuration, the auxiliary magnetization control layer additionally functions as an upper shield layer. On the other hand, the layer does not configure the read gap so that it is favorable for decreasing the read gap. A CoFe layer is inserted between the upper exchange coupling layer and the NiFe layer to strengthen the exchange coupling strength of the upper exchange coupling layer.
In order to completely remove the non-magnetic layer, a portion of the auxiliary magnetization control layer needs to be removed. However, removing the portion of the auxiliary magnetization control layer makes a remaining thickness of the auxiliary magnetization control layer thinner, and may decrease the exchange coupling strength through the upper exchange coupling layer. The auxiliary magnetization control layer has to be maintained at an appropriate remaining thickness. Therefore, an initial film thickness of the auxiliary magnetization control layer becomes thicker as a consequence.
Generally, in a pillar forming process, films are not formed in a pillar shape having uniform cross sections in a bottom up (thickness) direction. The films are formed in a conical trapezoidal shape having a flared bottom portion, or in a shape whose cross section is trapezoidal. If the initial film thickness of the auxiliary magnetization control layer is increased, the width of the pillar portion of two magnetization free layers would become thick. Therefore, it may restrict a track per inch (TPI) in a track direction of the head. Accordingly, the initial film thickness of the auxiliary magnetization control layer is more desirable when the thickness is as thin as possible. However, as described above, there is a limit in view of the exchange coupling strength.
If conditions of the photoprocess were reconsidered, it would be possible that a resist layer is disposed directly on the auxiliary magnetization control layer. However, in this case, a surface of the auxiliary magnetization control layer is oxidized, and the oxidized surface of the auxiliary magnetization control layer needs to be removed. Therefore, in either case, a surface layer portion of the auxiliary magnetization control layer needs to be removed, and the auxiliary magnetization control layer has to be formed thicker in advance, so that a similar problem occurs.